a. Field of the Invention
The instant invention relates to ultrasound medical procedures. In particular, the instant invention relates to a lens-directed high intensity focused ultrasound (HIFU) transducer.
b. Background Art
High intensity focused ultrasound (HIFU) is a technique wherein thermal therapy is typically delivered to a patient in the form of a focused high power acoustic beam emanating from an acoustic transducer. The principle advantage of focusing in HIFU is that the heating beam can be focused to selectively treat tissue regions, including remote interior tissues. Thus, HIFU is actively being developed for many treatments, such as cardiac ablation to treat cardiac arrhythmia and the destruction of cancerous tissues at depth.
One challenge in the design of reliable HIFU transducers is placing the therapeutic heat at the desired location without allowing acoustic heating or secondary, loss-related heating to damage the transducer or non-targeted tissues or to interfere with the transducer's ongoing acoustic contact with tissue. Design of a HIFU transducer should therefore take into consideration both the heat created by the primary therapeutic acoustic energy directed upon or into the tissue and the waste heat generated in the transducer due to imperfect (e.g., less than 100%) electrical-to-acoustic energy conversion. One solution to this problem is to utilize a fluid-filled standoff membrane, which acts both as a physical acoustic standoff and as a thermal sink for heat near the transducer face. The fluid in the standoff may also be flowed or permitted to weep in order to further cool the tissue and/or to enhance tissue acoustic coupling.
There are three possible options for the construction of HIFU transducers. First, a mechanically focused transducer, with a shaped piezoemitter, may be employed. Second, a lens-focused transducer, with a generally flat piezoemitter, may be employed. The term “lens” refers to an acoustically-redirecting entity through which acoustic energy passes and which provides a useful beam direction or reshaping, for example by focusing the acoustic energy to one or more distal foci. Finally, an electronically focused transducer, generally incorporating a generally flat piezoemitter, which may include multiple separately-activatable or phase-delayed piezo subelements, may be utilized. All three options may further include an optional acoustic matching layer.
Virtually all extant acoustically focused therapeutic and surgical transducers are mechanically focused utilizing shaped piezoemitters. Such transducers are easy to design, of very high electroacoustic efficiency because of the lack of acoustically lossy materials in the beam path, and have negligible beam side-lobes because they typically have only a single piezoelement. They are, however, quite expensive to manufacture due to the complexity involved in shaping and surface-finishing non-flat piezoelectric materials.